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|15th July 2005, 10:48 AM||#1|
Join Date: Feb 2005
Bell Sound Mono Integrated Amplifier
I have recently refurbished a pair of Bell Sound 2325 mono integrated HiFi amplifiers for a friend. His neighbor used them for a while and then stuck them in the garage for the past 15 years or so! When he found out we were building tube amps, he gave them to us. Needeless to say, they needed some TLC to get back to peak working condition.
The BellSounds have....
-pair of P-P 6L6GB's with about 400v on the plates run in pentode mode for 20 watts out.
-decently filtered B+
-conservative tube operation
-baxandal stack in a feedback loop
-loudness, trebble an rumble cut filters
-cathode feedback for the output stage!
-individually adjustable cathode bias with test jacks
-4, 8, and 16 ohm taps
-DC filaments on the input buffer and phono stage (derived from the cathode current of the 6L6's)
-phono preamp with several curves.
The amps themsevles are almos identical, but the serial numbers are not near consecutive, and there are some differences. One unit has vent holes in the cover by the output and recto tubes. One has that big 8K screen resistor mounted on the chassis, the other has it under the chassis. One of the amps has had a few electrolytic caps replaced, used the regular molded paper caps to the output tube grids, and one of the output tube coupling caps was replaced with a "DURAMITE". It looks to me like someone had a mono setup and then bought a second amp to go stereo.
These amps look quite old, either late 50s or early 60s. They look like they are from the age before stereo. They also seem to have been serviced in 1969, as there is a date and (consecutive) serial number written on the bottom of both units which does NOT look like it came from the factory that way. Also one unit had a few capacitors (crummily) replaced. Looks like they were serviced after they became a stereo pair.
The electrolytic caps were total trash, all of the paper caps had small leakage (a couple of volts on the next stage), the replacement "DURAMITE" felt it would rather be an 8K resistor....
I replaced all the electrolytic capacitors and coupling capacitors. The coupling capacitors were replaced with "orange drop" style capacitors of equal or greater voltage handling and the same value. The .33 cap grounding the non-input side of the LTP got a tubular yellow capacitor since that was more redilly available. I replaced every coupling capacitor. no ceramic caps were replaced. I did not replace the caps in the loudness circuit, nor the caps in the rumble filter since they don't have DC on them, and they are out of the circuit most of the time anyways. The caps to the output tubes (mounted sideways) are a total PITA to replace. I replaced the duramite and molded paper output caps. The other unit already had orange drops here from the factory and they testd fine so they stayed. The capacitors in the phono EQ section (that have parallel resistors and no voltage accross them) stayed since they have no voltage on them and are parallelled by resistors. I might replace the ones used in the "RIAA" position, though. I removed the capacitor across the power switch.
Every single electrolytic cap was replaced. I upped the values of some of these capacitors. All B+ caps were replaced with 450volt Illinois capacitor or NTE (the screen cap). I used 47uF for the rectifier node and plate nodes. The screen node got a 33 Uf cap. The first three B+ nodes (PI, tonestack, and input buffer) were replaced with 22uF caps. The last 16 uF cap for the phono stage was replaced by two 10 uF caps since I cleaned out my neighborhood parts place of the 22's! The phono stage got a 10 uF cathode resistor since I wasn't sure if it imparted a designed in rolloff. The NFB coupling capacitor and output stage cathode bypass capacitor were replaced with 220 uF or larger units.
All tubes and pots were good and simply needed a good cleaning for absolutely perfect operation. One resistor had to be changed. I'm talking to my friend about trying some new 6L6's and messing around with some different 12AX7's
I generally agree with the design choices of this circuit and I absolutely love Cathode feedback output stages! I just wish they replaced that stupid 12AV6 with a 12A_7 for easier replacement, and just used the other triode section to buffer the tape out or something. I think these things sound absolutely fantastic!
My question to you is......
-Have any of you ever heard of or had any experience with Bell Sound Amplifiers or similar?
-Is there anything you would recommend to improve the performance of these units (nothing too major, I'd like to keep these relatively stock)?
-What is your opinion of the Bell Sound amplifier circuit?
-Do you think I restored them correctly?
I was thinking about simply bypassing some of the electrlytic capacitors with film capacitors and then adding a zener diode (slightl larger than 12 volts) accross the heaters of the input buffer and phono stage to prevent damage to them should something happen to the 6L6 tubes.
Thanks and sorry for the overly long post!
|15th July 2005, 11:46 AM||#2|
Join Date: Oct 2002
Blog Entries: 2
Nice score! How big are the output transformers?
You'll no doubt get a wide range of opinions on circuit changes! If it were me, I'd probably start by measuring the OPT turns ratios, primary to secondary and between primaries, to see what my output tube options were. I'm totally not a fan of that sort of phase-splitter/driver circuit and the use of a 12AX7 there, but replacing that will mean some extensive work. I'm even less of a fan of the phono circuit, but that would take an even bigger rebuild to correct. If you're happy with the sound of the amp, why futz with it?
I would skip the zener idea. One thing worth looking at is putting in a regulated supply for the input tube heaters. Since they're in series, a CCS is appropriate. I wouldn't worry too much about protecting them- if the output tubes fail, you'll have bigger worries than some relatively cheap 12AX7/12AV6.
Additionally, unless you're a collector of very old records, you might want to hard-wire in the RIAA network, losing some (unreliable) switch contacts.
“Just as soon as we get our opposable thumbs on an interesting concept, the rest of our chimp brains hijack it for a drunken joyride down the interstate.”
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